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Terrapin Files

Diamondback terrapins face a variety of threats—from coastal development to crab fishing. A team of VASG-funded researchers are mapping terrapin habitat and threats to aid in the development of effective management strategies.

A small terrapin does not fit through an orange bycatch reduction device (BRD) on a crab pot. ©Janet Krenn/VASG

A small terrapin does not fit through an orange bycatch reduction device (BRD) on a crab pot. ©Janet Krenn/VASG

Virginia Marine Resource Bulletin
Volume 44, Number 2, Summer 2012
By Kate Schimel

When Randy Chambers approached resource managers about his plan to study diamondback terrapins (Malaclemys terrapin), they made a deceptively simple request. “They told us, ‘well why don’t you tell us how many turtles there are in the Chesapeake Bay?’ We had no way to do that,” Chambers recalls.

Randy Chambers, Donna Bilkovic, and Matthias Leu pose with a crab pot fitted with BRDs. ©Will Sweatt/VASG

Randy Chambers, Donna Bilkovic, and Matthias Leu pose with a crab pot fitted with BRDs. ©Will Sweatt/VASG

The diamondback terrapin is classified as a species of high conservation need in Virginia, and it is in decline throughout its range, which extends from Massachusetts to Texas. But before managers can act to conserve the terrapin, they need basic information about the turtles and where they live—information that no one has collected systematically in the Bay.

Chambers, director of the Keck Environmental Field Laboratory at the College of William & Mary, is part of a Virginia Sea Grant (VASG)-funded research team that has just finished collecting basic information about where terrapins live along the York River’s saltwater marshes. The team is using data about these habitats and nearby land development to build a model to predict where terrapins might live elsewhere in the Bay. This information will help target terrapin protection and research efforts to the most important areas.

A targeted approach is important, says Donna Bilkovic, the lead investigator on the project and a researcher at the Center for Coastal Resources Management (CCRM) at Virginia Institute of Marine Science (VIMS). Human activities, including shoreline development and crabbing, affect terrapin survival. By limiting conservation efforts to areas where these activities overlap with terrapin habitat, managers can reduce costs to industry and communities while effectively protecting the species.

Curiosity Killed the Terrapin?
There’s something about crab pots that attracts terrapins. Like crabs, smaller male or immature female terrapins crawl into crab pots and become trapped. Strangely, terrapins are just as likely to get caught in unbaited pots as in baited, active pots.
“I think the turtles are inquisitive,” Chambers suggests. “We put the traps in the water, without bait, and the turtles just swim into them.” The terrapins’ attraction to crab pots frustrated Chambers on one sampling trip when he pulled up a pot containing a mature female terrapin that seemed too large to have fit through the opening. He spent half an hour extracting the animal only to watch her climb right back in as soon as he let her go.

Unfortunately, this curiosity can kill. Traps are usually checked only once a day; however, terrapins must surface about every hour to breathe, so the trapped animals frequently drown. Often, terrapins follow each other into the traps, further increasing fatalities. One study in Georgia reported 94 terrapins in a single trap, and Chambers himself has caught as many as 7 in an unbaited trap.
One way to keep terrapins from making the fatal mistake of entering a crab pot is to attach a small plastic rectangle to the entrance of the pot. This bycatch reduction device (BRD) prevents terrapins from entering. Studies done at VIMS by Rom Lipcius and Rochelle Seitz indicate that the device effectively keeps terrapins out while still letting crabs in. BRD use is not mandatory although a voluntary program to promote their use is underway.

However, Bilkovic, Chambers, and their team are reluctant to advocate that BRDs be required on crab pots throughout the Bay; instead, they suggest using the devices only in places where it would matter most—where terrapins are likely to be found. Unnecessary requirements could hamper a local industry already faced with strict regulation.

Another important approach to protecting terrapins from being trapped in crab pots is the removal of derelict and abandoned pots that continue to catch and kill turtles, fish, and crabs long after the pots’ owners have moved on. More than 32,000 lost crab traps were removed from the Bay and its tributaries in Virginia during a 4-year program run by CCRM and the Virginia Marine Resources Commission (VMRC) that employed crabbers themselves to survey for and remove derelict pots.

Mapping Terrapin Habitat
Of the thousands of crab pots in the Chesapeake Bay, Chambers predicted that relatively few would overlap with terrapin habitat.
To confirm this assumption, the team had to figure out where terrapins live. Rather than survey the entire Chesapeake Bay, they examined a stretch of the York River shoreline for terrapins and gathered information about these places to start drawing broader conclusions about the habitats the animals prefer. They have found that healthy marsh habitats and nearby nesting beaches are crucial to terrapin success.

Terrapins tend to remain in the same area for their entire lives. On one beach in Rhode Island, observers have reported seeing the same female nesting over the course of 20 years. This consistency leaves terrapins vulnerable to localized habitat loss and predation.
Coastal development and shoreline hardening also threaten the marsh habitat where terrapins live. According to a report from CCRM, more than 11 percent of Virginia tidal waters have already been hardened with seawalls or other structures for erosion prevention and protection from sea level rise. The VASG-funded research team did not observe any terrapins in hardened areas.

Female terrapins are especially vulnerable to the effects of development since they have to leave the marshes to nest. Erosion and sea level rise eat away at nesting beaches, and increased development has resulted in large numbers of road-kill deaths among females, that cross roads while searching for places to lay their eggs.

Terrapin young are further threatened by increased predation on nests and hatchlings by raccoons and birds. “In developed areas there’s a link [between terrapin declines and] increased raccoon populations,” says Bilkovic. “Raccoons feed voraciously on terrapin eggs.” At one study site in the Goodwin Islands near the mouth of the York River, researchers recorded 87 percent mortality in terrapin nests due to predation.

A Model of Collaboration
The results of the VASG-funded study suggest that only about 15 percent of the area surveyed along the York is in conflict, meaning that it is used by both terrapins and crabbers. The next step for the team is to create a model that predicts terrapin distributions more broadly in areas that have not been directly surveyed for terrapins. Although the team collected turtle survey data in the field, much of the information that will shape the model comes from other sources.

For example, the team is using maps of coastal development to show where predation and habitat loss are expected to be greatest. Because many of these data sets already exist for other regions, the research team believes they can use their work to look beyond the York River and start predicting where terrapins might be found in other rivers and in other states.

“We’d like to create a model that could be extended to all of the Atlantic states,” says Bilkovic.

To develop such a complicated model, Bilkovic has assembled a team of experts in addition to herself and Chambers. Matthias Leu is a William and Mary conservation biologist and modeler and Timothy Russell is a William and Mary Geographic Information System (GIS) expert. Kirk Havens, CCRM’s Assistant Director, is leading the effort to recover derelict crab pots, will contribute data on where the pots have been found.

“We put all of these people together who have their different expertise, and I’m really fascinated to see how this product is going to develop,” says Chambers. “I think, collectively, we’ve covered all the different pieces you would need to make a model—a validated, good model. So I’m very excited about the project.”